Check on weight and height as these two are key indicators of puberty/maturity. Even though height is also determined by the breed, a dwarfed heifer will not mature at the right time.
At 14-15 months, when they are ready for service, a heifer should be 270-350kg bodyweight. Use of a good mineral supplement is key. Maclik Plus given from 6 months age helps to achieve musculo-skeletal growth and maturity of the reproductive system. Good nutrition (proteins, energy, vitamins and minerals) should be provided from weaning.